Within minutes of receiving the award, Dr. Birkar noticed that his briefcase containing the gold medal, his wallet and his cellphone was missing, the Brazilian newspaper O Globo reported.

The briefcase was later found outside the venue, but the medal was missing.

The organizing committee from the International Congress of Mathematics is analyzing recorded images of the event in Rio de Janeiro and working with the local authorities in their investigation to identify the thief.

Dr. Birkar, who was born and raised in Kurdistan Province in northwestern Iran, moved to England in 2000, after applying for political asylum while finishing his undergraduate degree in Tehran.

He completed a doctorate at the University of Nottingham, and developed theories that have solved longstanding conjectures, according to the University of Cambridge, where he now holds a professorship.

While studying at Tehran University, Dr. Birkar would look up at pictures of Fields Medal winners on the walls of his math club, he told Quanta magazine in an interview.

“I looked at them and said to myself, ‘Will I ever meet one of these people?’ At that time in Iran, I couldn’t even know that I’d be able to go to the West,” he said.

“To go from the point that I didn’t imagine meeting these people to the point where someday I hold a medal myself — I just couldn’t imagine that this would come true.”

Dr. Birkar is Cambridge University’s 11th Fields medalist.

“This is absolutely phenomenal, both for Caucher and for mathematics at Cambridge,” said Professor Gabriel Paternain, head of the university’s department of pure mathematics and mathematical statistics.

“Caucher was already an exceptional young researcher when he came to Cambridge, and he’s now one of the most remarkable people in this field.”

The other three winners of the 2018 Fields Medal are Alessio Figalli, of the Swiss Federal Institute of Technology in Zurich; Peter Scholze from the University of Bonn; and Akshay Venkatesh, of the Institute for Advanced Study in Princeton and Stanford University in California.

In addition to the medal, each receives an $11,500 cash prize.

Dr. Birkar won the award for his work on categorizing different kinds of polynomial equations, revealing that seemingly unrelated algebraic equations share something in common and can be classified into a smaller number of groups.

In his lifetime, the Kurds, a sizable minority in several countries, have suffered severe repression and the ravages of war in Iran, Iraq and Syria. In an interview on the Cambridge University website after receiving the award, Dr. Birkar said he had grown up in “an unlikely place for a kid to develop an interest in mathematics.”

“I’m hoping that this news will put a smile on the faces of those 40 million people,” he said.

Correction:Aug. 2, 2018

An earlier version of this article misspelled, in some instances, the surname of the professor whose Fields Medal was stolen. He is Caucher Birkar, not Birkir.

A version of this article appears in print on , Section A, Page 7 of the New York edition with the headline: Fields Medal Is Stolen Minutes After Ceremony in Brazil. Order Reprints | Today’s Paper | Subscribe